Has the found footage fad gone too far? That's a question often asked by people who don't know the half of it. Rather than fading away, the craze that started around 20 years ago has blended into the mainstream as people's home filmmaking equipment has improved so much in quality that we no longer see the rough edges as we used to. We might notice whether a film is in first person format or not, but even that has become less clear as we have grown used to observational films made by people who are undeniably present but not participating. America The Beautiful takes us back to something much more blunt and obvious. It's not the first film to be shot entirely on mobile phones - 2015 sensation Tangerine is widely believed to have that honour - but it is the first you're likely to encounter at a film festival that's shot entirely in that shape, as a narrow bar of action in the middle of the screen.

Isn't that annoying? For many viewers, yes, and there's no doubt that it will limit the size of the film's audience. It's also likely to leave you feeling cross-eyed after a while - most films are the shape they are because of the shape of our faces. But it does have some artistic purpose, firstly in making the events of the film feel rawer, closer to home; secondly in drawing the viewer in. There's a sense of voyeurism here, as if one were peeping through a keyhole or using an old What The Butler Saw machine. We are looking at events that everybody knows go on but that most people don't want to acknowledge being close to. We are seeing America without her clothes on.

This is how it happens. An ordinary man, a marketing manager, living with his girlfriend in the suburbs. Sometimes in the evenings he hangs out at small parties, and one day there's a Donald Trump supporter there, espousing views so far beyond what the others would normally contemplate that they don't even really offend - it's hard to believe that they're serious. He's upset but his girlfriend reminds him that they both knew such people existed, tells him not to worry about it. So he lets it go, but somewhere in his mind, a seed has been planted. Over time, he starts to consider perspectives that he hasn't really given time to before. When his girlfriend is late home he starts to wonder where she's been and whether he might be losing control of her. When people offend him he starts to notice their race, and soon he's being offended just by the way they look, by the heritage and attitudes he imagines they have.

This isn't a happy journey for him, any more than it is for anyone else. By the time he finds comradeship in a local militia he feels a need for it that wasn't there before; he's alienated everyone else, torturing himself with paranoia. His girlfriend, who undergoes no such transformation, grows increasingly distressed by the assumptions people make about her because they're together. She tries to reason with him but everything begins to feel like an attack, reinforcing his new sense of identity.

Though it's clear early on that this journey will lead to something brutal, directors James Kaelan and Blessing Yen exercise restraint, letting changes take place slowly. We observe them through arguments with other people, mostly the girlfriend; and through director observations made by the troubled man as he uses his phone to try and spy on imagined threats. Being so close to him might invite sympathy but his developing obsessions are just sufficiently out of balance wit what we actually see that it's clear something is going wrong with his thought process. US cinema loves its madmen but here there is nothing edgy or exciting about what develops, just the awful sense of loss that such things bring in real life. Furthermore, there is never any suggestion that he has ceased to be able to make choices, to take responsibility for his own actions.

Viewers who sympathise with the politics of Trump and his supporters might reasonably point out that not everyone with such views chooses to go down the same path as this man. That's not really the film's concern, however; its interest lies in what happens on the fringes of such movements, on what the removal of certain taboos makes possible. The film is stronger because it keeps its story small. Most of what we see is very ordinary, and we're invited to observe the damage done by small expressions of irrational resentment and hate, even when the protagonist himself doesn't grasp them.

Deliberately plain in its style, this film is not seeking great artistic status, but it is an important comment on the contemporary American experience and a wake-up call to anyone who imagines that such ideas could never take root in their world.